Confronting feedback processes on degraded coral reefs

Coral reefs are degrading under global stressors that are increasing in frequency and severity as the Anthropocene accelerates. My thesis contributes to our scientific understanding of the dynamics that govern degraded coral reef states. More specifically, I contribute to our understanding of feedback processes on degraded coral reefs in conceptual and experimental ways by confronting both ecological and social-ecological feedbacks in ways that may have merit in triggering coral recovery. My four presented studies (Chapters 1-4) pursue the following research questions: 1. Which habitat drivers best predict juvenile coral densities following bleaching? 2. Can macroalgae-reinforcing feedbacks be weakened through shading? 3. Can sea urchins effectively weaken macroalgal feedbacks given their current natural densities? 4. Can red and green loops uncover missing social-ecological feedbacks? Juvenile corals are a critical life history stage representing survival and growth of new recruits into the population. Chapter 1 compares juvenile coral densities from before the 2016 bleaching event with those after and identifies abiotic and biotic habitat drivers collected in the inner Seychelles that predict juvenile coral densities. Following the 2016 bleaching event, juvenile coral densities were significantly reduced by about 70 %, with a particularly severe decline in juvenile Acropora corals. Macroalgae present a major obstacle to survival of juvenile corals shortly following mass bleaching, but their influence varies as a function of herbivore biomass, reef structure, and reef type. In contrast, increasing structural complexity on granitic reefs is a strong positive predictor of juvenile coral density. Macroalgae can maintain and increase their dominance with effective self-reinforcing feedback mechanisms and can significantly compromise ecosystem function. Chapter 2 assesses shading as a management tool in an experimental confrontation of macroalgal feedbacks, aiming to maximise the benefit of habitat mosaic reefscapes in the inner Seychelles. Shading reduces the algae’s ability to photosynthesise by 29 % to the point where macroalgal cover can be reduced by 51 % and turf algal growth can be reduced by 82 % within six weeks of shading. After removal of shading structures, herbivore grazing rates decreased at shading plots, and algal beds recovered quickly, almost completely regrowing within three months. Tropical sea urchins are often considered as macroalgal grazers, but this assumption relies heavily on geographically limited observations of select species. Chapter 3 addresses these gaps for a common urchin species in the Seychelles, Echinothrix calamaris, using a combination of survey and experimental approaches in the inner Seychelles. Habitat driver models revealed patch-reef types as the best positive predictor and macroalgae as the best negative predictor of urchin densities. Experimentally penning urchin densities (maximum 4.44 urchins m-²) resulted in a reduction of macroalgal cover by only 13 %. Therefore E. calamaris at current densities in Seychelles (mean: 0.02 urchins m-2, maximum: 0.16 urchins m-2) are unlikely to perform significant macroalgae controlling functions. People use their local ecosystems and can retrieve signals about how their actions affect ecosystem health. Capturing, interpreting, and responding to signals that indicate changes in ecosystems is key for their sustainable management and breaks in this signal-response, called missing feedbacks, will allow ecosystem health to degrade unnoticed. Chapter 4 applies an existing concept from sustainability science, the red-loop green-loop (RL-GL) model, to uncover missing feedbacks between reefs and people of Jamaica from the year 600 until now. This allowed the factors responsible for missing feedbacks to be identified – a main factor in Jamaica was seafood exports. An intervention to move Jamaica back to more sustainable dynamics between people and reefs could be to gradually move away from seafood exports and build ownership and management capacity in local seafoods. Overall, my thesis emphasises the importance of habitat for coral recruitment following severe coral bleaching as well as for urchin density and function in Seychelles. Furthermore, I cover management approaches to confront reinforcing feedbacks of expanding macroalgal fields, especially for a mosaic reefscape setting. I test the first method to reduce macroalgal cover via the alteration of the light regime. My thesis also includes the first study to apply the RL-GL concept to a coral reef social-ecological system and I advocate for its practicality in uncovering missing feedbacks and in gaining an understanding of past, present, and future sustainability that can be of use in other systems

Academic Engagement with Wadden Sea Stakeholders: A Review of Past Foci and Possible Futures

The Wadden Sea became a UNESCO World Heritage Site in 2009 owing to its geographical and ecological importance. Given its status and its global recognition, academic understanding of, and engagement with, a diverse set of stakeholders is crucial to the sustainability of the Wadden Sea and the wildlife that inhabit its transnational boundaries. As such, this paper reviews with whom, how, and to what extent the academy has engaged with Wadden Sea stakeholders. This study finds that stakeholder groups (whom, with vested interests in the sea, might be expected to be present) are missing from academic publications focused on stakeholders in the Wadden Sea. Moreover, existing studies tend to focus on singular, categorized stakeholder ‘groups’, and lack transboundary integration, as well as reference to UN Sustainability Goal 14 – a key target for environmental protection. In sum, the review provides (1) an analysis of academic work that engages Wadden Sea stakeholders to assist future researchers undertaking work in this global ecologically significant area, and (2) a discussion of where future academic work might be developed

Modeling drivers of biodiversity change emphasizes the need for multivariate assessments and rescaled targeting for management

The current policy and goals aimed to conserve biodiversity and manage biodiversity change are often formulated at the global scale. At smaller scales however, biodiversity change is more nuanced leading to a plethora of trends in different metrics of alpha diversity and temporal turnover. Therefore, large-scale policy targets do not translate easily into local to regional management decisions for biodiversity. Using long-term monitoring data from the Wadden Sea (Southern North Sea), joining structural equation models and general dissimilarity models enabled a better overview of the drivers of biodiversity change. Few commonalities emerged as birds, fish, macroinvertebrates, and phytoplankton differed in their response to certain drivers of change. These differences were additionally dependent upon the biodiversity aspect in question and which environmental data were recorded in each monitoring program. No single biodiversity metric or model sufficed to capture all ongoing change, which requires an explicitly multivariate approaches to biodiversity assessment in local ecosystem management

Temporal declines in Wadden Sea phytoplankton cell volumes observed within and across species

Cell size is a master trait in the functional ecology of phytoplankton correlating with numerous morphological, physiological, and life-cycle characteristics of species that constrain their nutrient use, growth, and edibility. In contrast to well-known spatial patterns in cell size at macroecological scales or temporal changes in experimental contexts, few data sets allow testing temporal changes in cell sizes within ecosystems. To analyze the temporal changes of intraspecific and community-wide cell size, we use the phytoplankton data derived from the Lower Saxony Wadden Sea monitoring program, which comprises sample- and species-specific measurements of cell volume from 1710 samples collected over 14 yr. We find significant reductions in both the cell volume of most species and the weighted mean cell size of communities. Mainly diatoms showed this decline, whereas the size of dinoflagellates seemed to be less responsive. The magnitude of the trend indicates that cell volumes are about 30% smaller now than a decade ago. This interannual trend is overlayed by seasonal cycles with smaller cells typically observed in summer. In the subset of samples including environmental conditions, small community cell size was strongly related to high temperatures and low total phosphorus concentration. We conclude that cell size captures ongoing changes in phytoplankton communities beyond the changes in species composition. In addition, based on the changes in species biovolumes revealed by our analysis, we warn that using standard cell size values in phytoplankton assessment will not only miss temporal changes in size, but also lead to systematic errors in biomass estimates over time

Thresholds and tipping points are tempting but not necessarily suitable concepts to address anthropogenic biodiversity change—an intervention

Thresholds and tipping points are frequently used concepts to address the risks of global change pressures and their mitigation. It is tempting to also consider them to understand biodiversity change and design measures to ensure biotic integrity. Here, we argue that thresholds and tipping points do not work well in the context of biodiversity change for conceptual, ethical, and empirical reasons. Defining a threshold for biodiversity change (a maximum tolerable degree of turnover or loss) neglects that ecosystem multifunctionality often relies on the complete entangled web of species interactions and invokes the ethical issue of declaring some biodiversity dispensable. Alternatively defining a threshold for pressures on biodiversity might seem more straightforward as it addresses the causes of biodiversity change. However, most biodiversity change appears to be gradual and accumulating over time rather than reflecting a disproportionate change when transgressing a pressure threshold. Moreover, biodiversity change is not in synchrony with environmental change, but massively delayed through inertia inflicted by population dynamics and demography. In consequence, formulating environmental management targets as preventing the transgression of thresholds is less useful in the context of biodiversity change, as such thresholds neither capture how biodiversity responds to anthropogenic pressures nor how it links to ecosystem functioning. Instead, addressing biodiversity change requires reflecting the spatiotemporal complexity of altered local community dynamics and temporal turnover in composition leading to shifts in distributional ranges and species interactions

Uncovering drivers of juvenile coral density following mass bleaching

Thermally induced mass coral bleaching is globally responsible for major losses of coral cover. Coral recovery from mass coral disturbances like the 2016 bleaching event hinges on successful recruitment of new coral colonies to the existing population. Juvenile corals as a life history stage represent survival and growth of new recruits. As such, habitat preferences of juvenile corals and how environmental parameters interact to drive coral recovery following a mass bleaching disturbance are important research areas. To expand our knowledge on this topic, we compared juvenile coral densities from before the 2016 bleaching event with those after the disturbance and identified abiotic and biotic characteristics of 21 reefs in the inner Seychelles that predict juvenile coral densities. Our results show that following the 2016 bleaching event, juvenile coral densities were significantly reduced by about 70%, with a particularly large decline in juvenile Acropora. Macroalgae present a large obstacle to survival of juvenile corals in a post-bleaching setting, but their influence varies as a function of herbivore biomass, reef structure, and reef type. Higher biomass of herbivorous fish weakens the negative effect of macroalgae on juvenile corals, and structural complexity on granitic reefs is a strong positive predictor of juvenile coral density. However, structural complexity on carbonate or patch reefs was negatively related to juvenile coral density, highlighting the importance of considering interactive terms in analyses. Our study emphasises the importance of habitat for juvenile coral abundance at both fine and seascape scales, adding to the literature on drivers of reef rebound potential following severe coral bleaching

Temporal declines in Wadden Sea phytoplankton cell volumes observed within and across species

Cell size is a master trait in the functional ecology of phytoplankton correlating with numerous morphological, physiological, and life‐cycle characteristics of species that constrain their nutrient use, growth, and edibility. In contrast to well‐known spatial patterns in cell size at macroecological scales or temporal changes in experimental contexts, few data sets allow testing temporal changes in cell sizes within ecosystems. To analyze the temporal changes of intraspecific and community‐wide cell size, we use the phytoplankton data derived from the Lower Saxony Wadden Sea monitoring program, which comprises sample‐ and species‐specific measurements of cell volume from 1710 samples collected over 14 yr. We find significant reductions in both the cell volume of most species and the weighted mean cell size of communities. Mainly diatoms showed this decline, whereas the size of dinoflagellates seemed to be less responsive. The magnitude of the trend indicates that cell volumes are about 30% smaller now than a decade ago. This interannual trend is overlayed by seasonal cycles with smaller cells typically observed in summer. In the subset of samples including environmental conditions, small community cell size was strongly related to high temperatures and low total phosphorus concentration. We conclude that cell size captures ongoing changes in phytoplankton communities beyond the changes in species composition. In addition, based on the changes in species biovolumes revealed by our analysis, we warn that using standard cell size values in phytoplankton assessment will not only miss temporal changes in size, but also lead to systematic errors in biomass estimates over time.Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Interreg V A program Deutschland‐Nederland of the European UnionNiedersächsisches Ministerium für Wissenschaft und Kultur http://dx.doi.org/10.13039/501100010570https://doi.org/10.5281/zenodo.579926

Habitat and fishing control grazing potential on coral reefs

Herbivory is a key process on coral reefs, which, through grazing of algae, can help sustain coral-dominated states on frequently disturbed reefs and reverse macroalgal regime shifts on degraded ones. Our understanding of herbivory on reefs is largely founded on feeding observations at small spatial scales, yet the biomass and structure of herbivore populations is more closely linked to processes which can be highly variable across large areas, such as benthic habitat turnover and fishing pressure. Though our understanding of spatiotemporal variation in grazer biomass is well developed, equivalent macroscale approaches to understanding bottom-up and top-down controls on herbivory are lacking. Here, we integrate underwater survey data of fish abundances from four Indo-Pacific island regions with herbivore feeding observations to estimate grazing rates for two herbivore functions, cropping (which controls turf algae) and scraping (which promotes coral settlement by clearing benthic substrate), for 72 coral reefs. By including a range of reef states, from coral to algal dominance and heavily fished to remote wilderness areas, we evaluate the influences of benthic habitat and fishing on the grazing rates of fish assemblages. Cropping rates were primarily influenced by benthic condition, with cropping maximized on structurally complex reefs with high substratum availability and low macroalgal cover. Fishing was the primary driver of scraping function, with scraping rates depleted at most reefs relative to remote, unfished reefs, though scraping did increase with substratum availability and structural complexity. Ultimately, benthic and fishing conditions influenced herbivore functioning through their effect on grazer biomass, which was tightly correlated to grazing rates. For a given level of biomass, we show that grazing rates are higher on reefs dominated by small-bodied fishes, suggesting that grazing pressure is greatest when grazer size structure is truncated. Stressors which cause coral declines and clear substrate for turf algae will likely stimulate increases in cropping rates, in both fished and protected areas. In contrast, scraping functions are already impaired at reefs inhabited by people, particularly where structural complexity has collapsed, indicating that restoration of these key processes will require scraper biomass to be rebuilt towards wilderness levels